The burning of fossil fuels, including coal, is necessary to meet the energy requirements of our society. However, the combustion of coal, and in particular, many lower grades of coal, produces sulfur oxides which are emitted to the atmosphere. The release of these compounds produces many detrimental environmental effects. Respiration of these pollutants can cause human health problems ranging from mild respiratory irritation to more serious chronic diseases. Sulfur oxides can also react with other compositions in the atmosphere to form acid precipitation which has the effect of acidifying bodies of water and destroying the wildlife which live in such habitats. Acid precipitation also can destroy manmade structures such as buildings and statues.
Industry has sought to burn coal with low sulfur content to avoid the problems associated with sulfur oxides emissions. However, such fuel is not always readily available and the costs to recover and transport such high quality coal is in many cases prohibitive. Therefore, to meet the objective of environmentally acceptable coal combustion, effective methods are needed to remove sulfur compounds from the coal before, during, and after combustion.
Recent revisions in the Federal Clean Air Act require a ninety percent reduction in pounds of sulfur dioxide per million Btu for high sulfur coal before release to the atmosphere of combustion byproducts for new sources of air pollution. Some states have applied stringent requirements for reduction of sulfur dioxide to existing facilities. Federal and state legislation, therefore, make it necessary to achieve high reductions in the amount of sulfur compounds emitted during the combustion of coal.
A method of reducing the sulfur content of coal before combustion includes: (1) grinding the coal to a small particle size to liberate the inorganic sulfur containing compounds and other ash forming minerals from coal; and (2) separating the inorganic material bearing sulfur from the organic portion, coal. A major limitation in this technique is that when coal is ground fine enough to liberate substantial quantities of sulfur minerals and ash-forming minerals, separation of the coal from the unwanted material and subsequent recovery of the coal become difficult.
The grind size required to enable a ninety percent pyrite reduction and eighty-five percent Btu recovery for most coals is less than 0.5 mm and frequently finer than 0.1 mm. At these sizes, reported beneficiation techniques are not consistently effective in separating coal at acceptable efficiencies.
Jigs, hydrocyclones and tables are inefficient for separation of minus 0.5 mm coal. Froth flotation is ineffective when applied to oxidized coals because their surface character is not sufficiently hydrophobic to be activated by collecting reagents. For unoxidized coals, good Btu recovery is attainable by froth flotation, but pyrite rejection is difficult because of the relative ease with which pyrite floats.
Ergun, U.S. Pat. No. 3,463,310 discloses a method of cleaning fine coal material (0.400 mm-0.037 mm) by subjecting a mixture of coal and pyrite to electro-magnetic radiation which selectively magnetizes pyrite. Pyrite is then removed by magnetic means. This process is limited to magnetizable refuse material such as pyrite. Other materials frequently found in coal, such as silica, cannot be removed by this method.
Dense media cyclones are efficient devices for separating coal in the quarter inch to 0.5 mm range from refuse material on the basis of coal and refuse material having different densities. A mixture of the two materials is suspended in a dense media to form a sink product and a float product. A dense media, or a psuedo-heavy liquid, is necessary because the specific gravities of coal and refuse material are greater than one, and therefore, cannot be separated by water alone. A media with an effective media specific gravity between that of coal and of refuse material is required. A common media useful for coal beneficiation is a suspension of magnetite particles in water. By introducing a coal-refuse material mixture into a magnetite media, clean coal floats and refuse material sinks. Separation of these materials is hastened by using a dense media cyclone which increases the nominal gravitational acceleration on the mixture.
The use of dense media cyclone separations for beneficiating coal is well known. For example, Miller, et al., U.S. Pat. No. 3,794,162, is directed toward a heavy medium beneficiating process for coal particles greater than 150 mesh (about 0.1 mm). Horsfall, U.S. Pat. No. 4,140,628, is also directed toward a dense medium separation process. Horsfall discloses the use of magnetite particles less than 0.100 mm for beneficiation of coal fines having a particle size less than 1.000 mm and, in particular, less than 0.500 mm. This process involves separation of materials in a suspension with a dense media to form two fractions and a series of subsequent screenings and washings of magnetite from the two fractions. Horsfall, however, does not address the question of efficiency of separation of the two products.
Previous attempts to extend the performance of dense media cyclones below 0.5 mm have generally met with limited success and, in particular, have been unsuccessful in terms of teaching a general method for efficient separation. One parameter which is useful in assessing the effectiveness of separation of coal fines and refuse material by dense media and other separation techniques is the Ecart Probable (Ep). The Ep value is defined as the difference between the particle density of that fraction of the cyclone feed having a 75% chance of reporting to the overflow minus the particle density of that fraction of the cyclone feed having a 25% chance of reporting to the overflow divided by two. The separation gravity is defined as the specific gravity of a small increment of the feed which reports fifty percent to the clean coal overflow and fifty percent to the refuse underflow. The Ep value is a measure of the sharpness or efficiency of the separation, while the separation gravity defines the specific gravity at which the separation occurred. This separation gravity is different for different size fractions of feed coal even though all size fractions are cleaned in the same dense media cyclone. Generally, a smaller size fraction has a higher separation gravity. Also, the specific gravity of the dense media is generally less than the separation gravity.
A typical dense media is a suspension of magnetite particles in water. The magnetite can be natural magnetite which has been milled. Magnetite is also recoverable from fly ash. For example, Aldrich, U.S. Pat. No. 4,432,868 discloses that magnetite particles less than 325 mesh in diameter, having 90% magnetics, and a specific gravity between 4.1 and 4.5, can be obtained from fly ash. Aldrich further discloses that such magnetite contains a high proportion of round particles which are desirable for heavy medium separation because round particles reduce the viscosity of the heavy medium and facilitate separation.
Fourie, et al., The Beneficiation of Fine Coal by Dense-Medium Cyclone, J. S. African Inst. Mining and Metallurgy, pp. 357-61 (October 1980), discloses dense media cyclone separation of a 0.5 mm-0.075 mm coal fraction in a heavy medium cyclone with milled magnetite with at least fifty percent less than 0.010 mm using a 150 mm diameter cyclone. Ep values from 0.020 to 0.031 were achieved. While acceptable separation efficiences were achieved by Fourie, et al., the reference does not address cleaning the minus 0.075 mm coal fraction or provide a general method for determining operational parameters necessary to achieve acceptable efficiences.
Extending the capability of density separation beyond reported limits to effectively separate coal fines smaller than 0.5 mm and particularly smaller than 0.075 mm is highly advantageous. Substantial reductions in sulfur content and high Btu recovery can be achieved with such coal sizes. The ability to clean such fine coal is also economical because waste coal fines which were previously unrecoverable can now be used as an additional fuel source. Accordingly, there is a need for an improved process for the beneficiation of minerals to effectively recover fine coal.